Normal conditions: what a balanced Pacific looks like

To understand El Niño, it helps to know what a normal year looks like across the tropical Pacific Ocean.

The winds. In a normal year, steady trade winds blow from east to west along the equator. They push warm surface water towards Indonesia, where it piles up into a deep "warm pool" (NOAA PMEL).

The ocean. Because warm water is pushed west, the warm layer near South America is thin. The boundary between warm surface water and cold deep water, called the thermocline (scientists often use the depth of 20°C water), slopes up towards the east. There, cold, nutrient-rich water rises to the surface. This is called upwelling, and it forms a "cold tongue" of cooler sea.

The air. Over the warm west, moist air rises, builds tall clouds and brings rain. High up, the air flows east and then sinks over the cooler east Pacific, where skies are drier. This loop is the Walker circulation. It is named after Sir Gilbert Walker, a former head of India's weather service, who studied the related pressure see-saw in the 1920s (IMD).

Why it matters for El Niño. El Niño begins when this balance breaks. The trade winds weaken, the warm water spreads east and the rain moves with it (NOAA Climate.gov). Knowing the normal pattern makes every El Niño map easier to read.

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